Positron emission tomography (PET) has gained attention as being effective in making an early diagnosis of cancers, cerebrovascular disorders, dementia and others. PET is a method for injecting a compound labeled with a trace amount of a positron emission nuclide to detect annihilation radiation emitted from the body, thereby imaging of metabolic functions such as sugar metabolism and examining the presence or absence of a disease and the seriousness of a disease. For the implementation thereof, PET scanners have been put into practical use.
The principle of PET is as follows. Positrons emitted from a positron emission nuclide by positron decay undergo pair annihilation with electrons in the vicinity, and the thus generated pair annihilation radiation at 511 keV is determined by a pair of radiation detectors according to the principle of coincidence. Thereby, the position at which the nuclide is present can be localized on one line segment (coincidence line) connecting between the pair of detectors. When an axis from the head of a patient to the feet is defined as a body axis, a distribution of the nuclide on a planar surface intersecting perpendicular with the body axis is obtained by image reconstruction in two-dimensional mode from data of the coincidence line determined on the planar surface in various directions.
Therefore, earlier PET scanners were constituted with single ring-type detectors in which detectors were arranged on a planar surface which was given as a field-of-view densely in a ring shape so as to surround the field-of-view. Thereafter, with the advent of a multiple ring-type detector in which many single ring-type detectors were densely arranged in the body axis direction, a field-of-view in two-dimensional mode was changed to that in three-dimensional mode. Further, in the 1990s, the coincidence was also determined between the detector rings to develop 3-D mode PET scanners one after another with a great increase in sensitivity. This trend is found even now.
In order to increase the sensitivity of a PET scanner, as illustrated in FIG. 1(a), it is necessary that detectors are arranged densely in a tunnel shape to constitute a multiple ring-type detector 10, thereby increasing a solid angle. However, a long tunnel-shaped patient port not only causes increased psychological stress to a patient 6 under examination but also affects medical care of the patient. In order to cope with this problem, as illustrated in FIG. 1(b), the applicant has proposed an open-type PET scanner in which multiple ring-type detectors 11, 12 which have been divided into plural regions in the body axis direction of a patient 6 are arranged apart to have a field-of-view region (also referred to as an open field-of-view) which is physically opened. In an open region, as shown in FIG. 2, an image is reconstructed from remaining coincidence lines between the multiple ring-type detectors 11, 12. In this drawing, the numeral 8 depicts a bed.
Here, as shown in FIG. 3, when a dimension (also referred to as a width) of each of the multiple ring-type detectors 11, 12 in the body axis direction is given as W and a dimension of the open region in the body axis direction (also referred to as a clearance) exceeds W, a region which can be imaged is discontinued in the body axis direction. Therefore, as shown in FIG. 3(a), a maximum value of an open-region clearance for obtaining a field-of-view continuing in the body axis direction is given as W. In this instance, a whole field-of-view in the body axis direction is given as 3W. However, a drastic reduction in sensitivity is caused at both ends of the open region.
Therefore, as shown in FIG. 3(b), the open-region clearance is given as αW (0<α≦1) to overlap the sensitivity, thus making it possible to prevent a reduction in sensitivity at both ends of the open region. In this instance, a field-of-view in the body axis direction is given as (2+α) W. As α is made smaller, the reduction in local sensitivity is suppressed accordingly, while the open-region clearance and the field-of-view in the body axis direction are reduced (refer to Taiga Yamaya, Taku Inaniwa, Shinichi Minohara, Eiji Yoshida, Naoko Inadama, Fumihiko Nishikido, Kengo Shibuya, Chih Fung Lam and Hideo Murayama, “A proposal of an open PET geometry,” Phy. Med. Biol., 53, pp. 757-773, 2008.).
In the open-type PET scanner previously proposed by the applicant, maximum values of an open-region clearance and a field-of-view in the body axis direction are respectively limited to W and 3W. Therefore, in order to further enlarge the open-region clearance and the field-of-view in the body axis direction, it is necessary to enlarge W itself. However, there is a problem that an increase in the number of detectors constituting one multiple ring-type detector makes the scanner more expensive and complicated.